The present invention relates to the non-intrusive passive acoustic detection and measurement of the change in wall thickness in a coker reactor.
It is often desirable to determine the change in wall thickness of a reactor that processes materials. The change in thickness may be erosion of the interior wall of the vessel such as a fluidized catalytic cracking unit or material buildup on the interior wall of the vessel such as wall coke on the interior wall of a fluidized bed coker.
Coking is a thermal process for converting heavy, residual oils into lighter products and solid carbon. In the earliest coking process, called delayed coking, after heating and partial vaporization, the residuum is passed into a coking drum which fills up with solid coke deposits. This coke must then be drilled out. See, e.g., U.S. Pat. No. 4,410,398. In an alternate process, the fluid coking process, coke is deposited on particles of seed coke in a fluidized bed and the coke product is in the form of freely flowing granules. Fluid coking also employs two beds with particles circulating between the coking reactor and a burner vessel where some coke particles are burned to produce the necessary heat.
Fluid coking is sensitive to feed flow and reactor temperature. If the heavy residual oil is fed too fast and the reactor is at too low a temperature, the coking reaction rate will be too low and coke particles will become wetted with incompletely reacted feed which increases their tendency to stick together in large poorly fluidizable lumps and to stick to the vessel wall producing wall coke. Correct control of feed rate at sufficiently high temperatures is necessary to prevent this bogging. We can define a critical bed temperature, the bogging temperature, T.sub.B, which will be a function of coker geometry, feed injection rate and character of feed. Currently fluid coker reactors are operated at temperatures far above the bogging temperature in order to avoid wall coke. High temperature operation favors the production of coke and light gases at the expense of more desirable liquid products. As a result, the yield of desirable liquid products is significantly reduced compared to lower temperature operation. Reducing the operating temperature for a particular unit requires the ability to determine when appreciable quantities of wall coke are being deposited.
Thus, there is a need for a rapid response reliable wall coke detector to monitor coke buildup and which would allow operation in the optimum range for liquid product yield. In addition to operating the reactor at temperatures closer to the bogging temperatures, there are other process parameters affecting the operation and product yield that need to be monitored. These include the pressure and velocity of the fluidizing gas (see U.S. Pat. No. 2,788,312).